18 IOWA ACADEMY OF SCIENCES. 



3. The difierence in the ease of erosion between g:lacio-natant drift clays and 

 the formations bordering them, may produce marked changes in topography and 

 drainage. ^Jhe regions of Dakota and Nebraska illustrate this well. Loose 

 sands and easily eroded clays border the western edge of the compact and often 

 boulder clad drift clays. While the latter are little affected by rains and stream- 

 lets, the former are rapidly removed. Moreover the former are peculiarly subject 

 to sliding and slushing out at base The amount of erosion which has taken place 

 since the occupation of the earlier glacial lakes may be more perfectly realized, 

 when we learn that the prominent high terrace found flong the Missouri, White 

 and Cheyenne rivers is more than 300 feet above their present water level. This 

 terrace dates from the time of the second moraine, or possibly of the first, of the 

 "second glacial epoch." And this terrace is mucli more recent than the lakes 

 under consideration. An erosion, which has excavated these valleys to such a 

 depth, must certainly have greatly changed the surface along the old lake borders. 



4. Yet another influence may have frequently done much to mask lacustrine 

 features, viz., orographic changes. Gilbert has recognized as prominent in the 

 cases of lakes Bonneville and Ontario. Chamberlin finds an elevation of Cham- 

 plain deposits, of 330 feet in eastern Wisconsin, and of -5-600 feet in northwestern 

 part of the same State. And this has been in a much less time than has elapsed 

 since Lake Missouri was filled with loess. 



So much on general principle. As may be remembered the writer has held 

 that the extra-morainic drift of the Missouri valley is probably of sub-aqueous 

 origin, that Lake Missouri which deposited the loess, at an earlier stage was 

 partly filled with sand, gravel and boulder clay; that a similar lake occupied the 

 Red Lake region, from the Bijou Hills to the Big Bend. Also that a similar one 

 covered a w^ide scope of country from near the mouth of the Moreau northward. 

 Hitherto, 1 have found rather scanty evidence of an old water level in the distribu- 

 tion of boulders about the Bijou Hills, 590 above the Missouri or 1.900 above the 

 sea, and a patch of bouldery gravel and clay 510 feet above the Missouri, covering 

 an acre or so, south of the mouth of White River. 



In 1888 Prof. G. F. Wright reported the finding of something like a moraine. along 

 the divide south of the Moreau river. (See Proc. A. A. A. S. 1888.) 



It has been my privilege the past season (1890) to traver&e the course of Prof. 

 Wright, with the same companion, Rev. T. L. Riggs, and to spend a few days in 

 the examination of this feature. 



I found Fox Ridge a high sandy plateau, forming the divide between the Moreau 

 and Big Cheyenne rivers. Upon it, and on its south slope, I found no northern 

 erratics. Its summit twenty miles west of the Missouri is about 2,400 feet above 

 the sea. Along its northern slope is a peculiar flat-topped, butte-like ridge run- 

 ning east and west for 15-20 mile.s, its top being nearly horizontal and about -50 

 feet lower than the summit of Fox Ridge. 



This was determined not only by several barometric readings, but by distant 

 views from both north and south. The ridge is well coyered with granite boulders, 

 and drift 2-5 feet thick, but strange to say no nortliern drift was found south of 

 the ridge, except where its presence could be accounted for by recent transporta- 

 tion. The land just south of the ridge is frequently 50 feet lower than its top. 

 This ridge is not strictly continuous. There is a wide gap, particularly, where it 

 is crossed by the Virgin creek. 



The margin of the drift I had not time to trace fully, but was informed by Mr. 

 Riggs, who knows the region well, that it crosses the Moreau 25-30 miles west of 

 its mouth and runs northward at about the same distance from the Missouri for an 



